need some help with setting permissions to folders

Hi in my office we are trying to setup different users in our domain with privileges on some shard folders. basically we have a common network scan folder that all users in the domain need to have access to. inside this folder are some sub folders example : accounting. we only want 2 people to have access to this folder. when I log in and go to set permissions instead of a list of usernames I see the domain user group. and if I set permissions it would set them for the group not the individual users in the group. How do I set the special permissions for only certain users in the domain group? thanks